During the planting time of corn, triazine-containing herbicides such as atrazine and chloroacetoanilide-containing herbicides such as alachlor and metolachlor have been conventionally used. However, atrazine shows low efficacy to grass weeds, and on the other hand, alachlor and metolachlor show low efficacy to broad-leaved weeds. It is therefore difficult at present to control grass weeds and broad-leaved weeds together simultaneously with a single herbicide. Further, even a mixture of these herbicides produces no sufficient herbicidal efficacy, and these herbicides are undesirable in view of an environmental problem due to their high dosage requirement.
On the other hand, during the planting time of cotton, trifluralin, a dinitroaniline-based herbicide, fluometuron, a urea-based herbicide and norflurazon, a pyridazine-containing herbicide have been used as herbicides for pre-emergence treatment. Further, during the planting time of wheat or barley, urea-containing herbicides such as isoproturon and chlorotoruron have been used for controlling grass weeds in particular. Since, however, these herbicides require highdosages for sufficient herbicidal efficacy, these are also environmentally undesirable.
It is known that specific isoxazole derivatives have herbicidal activity (JP-A-5-255284, WO94/18179).
Compound 2 disclosed in WO94/18179, a typical example of the isoxazole derivatives disclosed in the above publications, has the following structural formula. ##STR2##
Further, a herbicidally active compound having a thiochroman ring is already known as a pyrazole derivative in which a thiochroman ring and a pyrazole ring are bonded to each other, and International Laid-open Patent Publication WO93/18031 of PCT application filed by Applicant discloses, for example, a compound (Compound No. 68) of the following formula. ##STR3##
Further, there has been published the following laid-open publication including herbicide compositions having structures closer to the isoxazole derivatives of the present invention. That is, EP95/0636622A1 (JP-A-7-149742) discloses isoxazole derivatives having a structure in which isoxazole and a benzo-condensation type dicyclic group bond to each other and having herbicidal activity, and it describes a thiochroman ring which is part of the skeleton of the compounds of the present invention. However, the above publication describes no specific "Example" concerning a compound having a thiochroman ring. The above publication in Examples discloses compounds having a benzo-condensation type dicyclic group structurally similar to a thiochroman ring, for example, a compound (Compound No. 25) of the following formula. However, these compounds are practically insufficient in efficacy as herbicides. ##STR4##